This application is based upon, and claims the benefit of priority of prior Japanese patent Application 2001-161094, filed on May 29, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a vehicle headlamp using a discharge lamp and a controlled air flow through the headlamp to cool the discharge lamp and associated controls.
2. Description of Related Art
Generally, a discharge lamp has been used as a vehicle headlamp. The discharge lamp is an arc-discharge type electrical lamp. In the arc-discharge type lamp, xenon gas, or other appropriate gas, is enclosed in a discharge lamp bulb (arc tube), and light is emitted from the discharge lamp bulb using charge generated between a pair of electrodes therein. The light emitted from the discharge lamp bulb is white light similar to sun light. A quantity of light from the discharge lamp can be equal to or more than twice as much as that from a conventional halogen lamp while also using about 30% of the electrical power commonly used by a halogen lamp. Accordingly, when the discharge lamp is used as a vehicle headlamp, visibility forward of the lights is improved. That is, passengers will be able to see further in front of the vehicle compared to the use of halogen lights, with all other parameters being equal.
Generally, when the discharge lamp is turned on, a high voltage of several kilovolts (kV) to tens of kV is applied to the discharge lamp, thereby instantly beginning discharge therein, and lighting the light. Thereafter, electric power of about 35 watts (W) is applied thereto, thereby maintaining the lit condition. Therefore, a dedicated lighting-control circuit is required to light the discharge lamp. A lighting control circuit is mainly constructed of a converter, an inverter and an igniter. The converter increases direct-current (DC) voltage from a battery, and the inverter converts DC voltage to an alternating-current (AC) voltage. The igniter generates high voltage on the order of tens of kV.
In a vehicle, the lighting control circuit is generally mounted outside the lamp housing of a headlamp. In this case, wires, usually high voltage wires, and connectors, used for applying a high voltage to the discharge lamp, are required, thereby not reducing a size of the headlamp and its peripheral circuit. Further, many labor hours for attaching and wiring the high voltage wires and the connectors are required. Additionally, vehicle space for the wires and the connectors are required. Furthermore, high voltage wires require shielding for reducing adverse effects of electrical noises (electrical interference) from the wires, thereby increasing production costs of the headlamp and its peripheral circuit.
Therefore, it can be considered that the lighting control circuit should be mounted directly on the discharge lamp bulb in the lamp housing. For example, it is conceived that the discharge lamp bulb is connected directly to a connector contained in the lighting control circuit while the lighting control circuit is fixed to a rear surface of a reflector. According to this arrangement, no high voltage wires are required between the discharge lamp bulb and the lighting control circuit while the size of a vehicle headlamp can be reduced, thereby eliminating radio wave noises and power losses due to the high voltage wires.
While the discharge lamp bulb is lit, its temperature will increase and become high. Generally, ventilation holes are provided in the lamp housing to eliminate any pressure difference between the interior and the exterior of the lamp housing. However, each ventilation hole is set to a minimum area to prevent water and foreign matter from entering the lamp housing, so that almost no air stream or current is generated in the lamp housing. Therefore, while the discharge lamp bulb is lit, the air temperature in the lamp housing becomes high. That is, the temperature of the lighting control circuit becomes high by receiving heat from the discharge lamp bulb through thermal conduction, thermal radiation, and convection while the air temperature around the lighting control circuit becomes high. Thus, it is feared that excessive thermal loads are applied to circuit elements contained in the lighting control circuit. Although circuit elements and a circuit board, each having a high thermal resistance may be used as a remedy, production costs of the lighting control circuit and a size thereof increase with such requirements.
The present invention has been made in view of the above problems. Therefore, it is an object of the present invention to provide a vehicle headlamp to prevent the lighting control circuit temperature from increasing. That is, the lighting control circuit will be cooled by directing outside air into the lamp housing using the temperature difference of air between an interior and an exterior of the lamp housing. That is, mainly by using convection currents generated as a result of the temperature difference. Furthermore, it is an object to maintain cooling while preventing water droplets and debris from entering the lamp housing.
In embodiments of the present invention, the following technological means are adopted to attain the above object. In a vehicle headlamp according to the present invention, a lamp housing includes a first opening lower than a lighting control circuit and a second opening higher than the lighting control circuit. Further, the first and second openings are opened toward an internal space where the lighting control circuit is contained. The space is naturally ventilated using the first and second openings. Therefore, the lighting control circuit can be cooled using a natural stream of air, that is, natural convection currents of air generated by the air temperature difference between an interior and an exterior of the lamp housing while a discharge lamp bulb is lit. Accordingly, the lighting control circuit temperature can be prevented from increasing, and thermal loads applied to the circuit elements can be reduced. Furthermore, the addition of a ventilation unit such as an electric fan is not required, thereby preventing production costs from increasing.
Additionally, an inhibition member, for preventing water droplets and dust (debris) from entering the space, is provided for either or both of the first and second openings. The inhibition member may be a filter for trapping water droplets and dust. Alternatively, the inhibition member may be a labyrinth communicating with any one or both of the first and second openings. Therefore, water droplets and dust can be prevented from entering the lamp housing without increasing the number of components and production costs.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.